Exhaust gas generated by combustion of fossil fuels in furnaces, ovens, and engines contain, for example, nitrogen oxides (NOX, including mainly NO and NO2), unburned hydrocarbons (HC), and carbon monoxide (CO), which are undesirable pollutants. Vehicles, e.g., diesel vehicles, utilize various pollution-control after treatment devices (such as a NOX absorber(s) and/or Selective Catalytic Reduction (SCR) catalyst(s)), to reduce NOX. For diesel vehicles using SCR catalysts, NOX can be converted to N2 and H2O by using ammonia (NH3) gas. However, NO and NO2 have very different reaction rates within an SCR converter. Additionally, the presence of NH3 can also interfere with various types of NOX sensors, thereby reducing their accuracy. In order for SCR catalysts to work efficiently and to avoid pollution breakthroughs, more effective control systems are needed. The development of more effective control systems requires commercial NOX sensors with improved accuracy and sensitivity for a particular NOX constituent species and with reduced susceptibility to cross-interference from other NOX constituents and from NH3.
For example, existing NOX sensing materials having the chemical composition of the general form (AB)2O4 or(AB)O3, which include stoichiometric amounts of A, where A is metal elements capable of +3 valence state, and B, where B is Fe or Cr, are sensitive to NOX. They are used as electrode materials in electrochemical devices for NOX sensing by virtue of the fact that they generate an electromotive force (emf) when exposed to NOX, the magnitude and polarity of which may be characterized using the non-equilibrium Nernst Equation. However, generally, they show cross-sensitivity to NH3 which tends to limit their usefulness to applications where NH3 is not a concern. In addition, the individual contributions of NO and NO2 to their NOX electromotive force (emf) outputs cannot readily be isolated. As such, while useful in some applications, these sensors are not generally suitable for applications that require NO2 sensors having minimum cross-interference with NO and NH3, or relatively larger NO2 emf outputs (i.e., improved signal-to-noise performance) or both.
Thus, cost effective NO2 sensors having reduced cross-sensitivity to NO and NH3, or relatively high emf outputs or both, that can reliably sense NO2 under exhaust gas conditions would be desirable for use in various NO2 control systems.